Heretofore, separation or concentration of a gas or liquid mixture that contains an organic substance is carried out by distillation, azeotropic distillation, or solvent extraction/distillation or with an adsorbent, etc., in accordance with the properties of the substance to be separated or concentrated. However, these methods have a drawback in that the methods require much energy or the methods are applicable only to a limitative range of substances to be separated or concentrated.
Recently, as a separation method capable of being an alternative to these methods, a membrane separation/concentration method has been proposed in which a membrane such as a polymer membrane or a zeolite membrane is used. Polymer membranes such as, for example, flat membranes and hollow fiber membranes have excellent workability but have a drawback in that the heat resistance thereof is low. In addition, polymer membranes have poor chemical resistance, and many polymer membranes swell upon contact with organic substances such as, in particular, organic solvents and organic acids. Consequently, polymer membranes are applicable only to a limitative range of substances to be separated or concentrated.
Meanwhile, zeolite membranes are usually used for separation or concentration as porous support-zeolite membrane composites which each include a porous support and a filmy zeolite formed thereon. For example, a mixture of an organic substance and water is brought into contact with a porous support-zeolite membrane composite to make the water selectively pass therethrough. Thus, the organic substance can be separated or concentrated.
Compared to separation by distillation or with an adsorbent, the separation or concentration using a membrane of an inorganic material such as a zeolite not only can reduce the amount of energy to be used but also can be carried out in a broader temperature range than with polymer membranes. Moreover, such inorganic membranes are applicable to separation of a mixture that contains an organic compound
Hitherto, A-type zeolite membranes (PTL 1) have industrially been used as zeolite membranes. A-type zeolite membranes have high hydrophilicity and high dehydrating ability. However, A-type zeolite membranes have an SiO2/Al2O3 molar ratio (hereinafter often called SAR) as low as 2 and hence have problems concerning water resistance and acid resistance. These zeolite membranes have been usable in limited systems. There have also been proposed: a method in which a mordenite type porous support-zeolite membrane composite is used for an alcohol/water mixture system to make the water selectively pass therethrough and thereby concentrate the alcohol (PTL 2); a method in which a ferrierite type porous support-zeolite membrane composite is used for an acetic acid/water mixture system to make the water selectively pass therethrough and thereby separate and concentrate the acetic acid (PTL 3); etc. However, the mordenite type porous support-zeolite membrane composite disclosed in PTL 2 and the ferrierite type porous support-zeolite membrane composite disclosed in PTL 3 have low permeation flux, and the throughputs therewith are insufficient for practical use.
A CHA-type zeolite membrane has been proposed as a zeolite membrane composite having an improved permeation flux (Non-PTL 1).
Furthermore, a CHA-type zeolite membrane that has a high SAR and is dense has been proposed as a membrane which is excellent in terms of acid resistance and water resistance, has a permeation flux sufficient for practical use, and has high separation performance (PTL 4).